Recently, with the spread of autofocus single-lens reflex cameras and prepackaged single use cameras, photography has become increasingly popular, and silver halide light sensitive photographic materials with expanded functions and lower price are desired. Accordingly, there is needed development of a silver halide photographic emulsion more compatible in manufacturing cost as well as enhanced photographic characteristics such as sensitivity, graininess and sharpness.
Silver halide light sensitive photographic materials are subject to various pressure during the manufacturing process and during their use. Photographic negative films for general use, for example, are often subject to large pressure in the process of cutting and perforating, and are bent or scratched when being transported through the camera. As is well known, when a variety of pressure is applied to the silver halide photographic material, variations of photographic performance result, and a technique of minimizing effects of such pressure is desired.
One of the dominant factors of enhancing sensitivity and image quality of silver halide photographic materials concerns silver halide grains and there has been energetically undergone development of silver halide grains with the aim of improving the sensitivity and image quality. To enhance the image quality, in general, it is effective to increase the number of grains per given silver halide amount by decreasing the grain size, leading to an increase of the number of color-developed points (that is, number of elements). However, a decrease of the grain size results in lowered sensitivity, so that it is limitative to satisfy both high sensitivity and enhanced image quality.
To realize further enhancement of sensitivity and image quality, there has been studied a technique for enhancing the ratio of sensitivity to grain size of each of the silver halide grains; and as one thereof is disclosed a technique of employing tabular silver halide grains in JP-A 58-111935, 58-111936, 58-111937, 58-113927 and 59-99433 (herein, the term "JP-A" refers to an unexamined and published Japanese Patent Application). The tabular silver halide grains each have a larger surface area per volume, as compared to silver halide regular crystal grains such as hexagonal, octahedral or dodecahedral grains, whereby a larger amount of a sensitizing dye can be adsorbed onto the grain surface, leading to enhancement of sensitivity, including an improvement in spectral sensitization efficiency.
Of the techniques for improving silver halide emulsions directed to enhancement of sensitivity and image quality, the most basic and important technique is one of preparing a monodispersed silver halide grain emulsion. Since there is a difference in the conditions for optimal chemical sensitization or spectral sensitization between grains with a larger size and those with a smaller size, it is difficult to undergo optimal chemical sensitization of a silver halide emulsion comprised of both larger grains and smaller grains, that is, an emulsion having a broad grain size distribution, often resulting in an increase of fog density or insufficient chemical sensitization. In the case of a monodispersed silver halide emulsion (with a narrow grain distribution), on the other hand, it is easy to provide an optimal chemical sensitization or spectral sensitization, enabling to prepare a silver halide emulsion with high sensitivity, lower fog and superior graininess.
As techniques for preparing a monodispersed silver halide tabular grain emulsion, JP-A 1-213637 discloses a technique of improving the sensitivity and graininess with monodispersed tabular grains having two parallel twin planes. JP-A 5-173268 and 6-202258 disclose preparation of a silver halide tabular grain emulsion with a narrow grain size distribution. It is noted that the techniques described above are directed to monodispersed silver halide tabular grains in which there is a small fluctuation with respect to the equivalent circular diameter between grains. As is well recognized in the art, the size of the tabular grains can be determined in terms of at least two parameters, that is, one is the equivalent circular diameter of the tabular grain, and the other one is the grain thickness. In other words, even if only the distribution with respect to the equivalent circular diameter of the tabular grains should be narrowed, the distribution of the tabular grains with respect to the grain size can not be narrowed. In cases where the distribution of the tabular grain size is broad, there arise problems such as deterioration of the graininess, due to fluctuations in sensitivity and differences in developability.
JP-A 6-258744 discloses a technique of improving sensitivity, contrast, pressure resistance, latent image stability by using monodispersed silver halide tabular grains having an aspect ratio of 2 or more and containing regions different in the halide composition within the grain, wherein the expression, monodispersed grains means those with a little fluctuation with respect to the equivalent circular diameter of the tabular grains. However, the technique described above does not include any reduction of the cost for manufacturing the silver halide tabular grain emulsion. JP-A 5-210188 discloses preparation of core grains containing 10 to 45 mol % iodide under the condition of maintaining the distance between grains of 0.1 to 3.0 .mu.m. (Herein, the above-described distance between grains divided by 0.89 is nearly identical to the mean distance between grains defined in the present invention, although both are differently defined.) However, the above technique does not include any intention of arbitrarily controlling the mean distance between grains in the process of grain growth nor concrete means therefor.
JP-A discloses a technique of introducing dislocation lines into tabular silver halide grains to enhance the sensitivity. As is generally known, application of pressure to silver halide grains results in fog or desensitization, and silver halide grains in which dislocation lines are introduced, exhibited marked desensitization when subjected to pressure. JP-A 3-189642 discloses a silver halide emulsion comprised of monodispersed tabular grains having an aspect ratio of 2 or more and dislocation lines of 10 or more in the fringe portions. However, it was proved that marked desensitization due to pressure, resulted from introduction of the dislocation lines, could not improved according to this technique.
In development of silver halide emulsions, an important viewpoint, besides photographic performance is the problem concerning the manufacturing cost. A advantageous method in the manufacturing cost is to increase the manufactured amount of each emulsion batch. In other words, it is to increase the yield of silver halide in a reaction vessel for preparing silver halide emulsion at the time of completion of grain growth. Thus, it is to increase the concentration of silver halide in the emulsion at the time of completion of the grain growth.
JP-B 59-43727 and JP-A 3-140946 disclose a technique of employing ultrafiltration to concentrate the volume of the reaction product (silver halide emulsion) in the process of preparing emulsions (herein, the term JP-B means an examined and published Japanese Patent). However, these disclosures do not suggest anything with respect to preparation of tabular grains, specifically, monodispersed silver halide tabular grain emulsion, nor intend to control a mean intergrain distance in the process of preparing a silver halide emulsion.
JP-A 6-67326 discloses the preparation of a silver halide tabular grain emulsion, in which ultrafiltration is employed to concentrate the volume of reaction products to enhance the yield and obtain tabular grains having medium aspect ratios. According to this disclosure, employing the phenomenon that the aspect ratio of the tabular grains is decreased with concentration, tabular grains having a medium aspect ratio can be obtained by concentrating a reaction mixture containing tabular grains having a high aspect ratio by ultrafiltration during the grain formation. In general, the higher the aspect ratio, the more difficult it is to obtain monodispersed tabular grains. Since it is generally difficult to prepare monodispersed tabular grains having a high aspect ratio, it can not be expected to obtain monodispersed tabular grains having a low aspect ratio by concentrating a high aspect ratio tabular grain emulsion. In fact, silver halide emulsions disclosed therein, which include inventive and comparative emulsions, exhibit a variation coefficient of an equivalent spherical diameter of 0.3 or more, whereby problems regarding to photographic performance of conventional tabular grains cannot be solved. Furthermore, the disclosure teaches nothing with respect to an apparatus for preparing a silver halide emulsion.